1. Field of the Invention
The present invention relates to a coke drum skirt connection, and more particularly to a connecting system designed to greatly reduce or eliminate the occurrence of low cycle fatigue stresses that typically manifest at and below the circumferential drum to skirt weld of a delayed coker drum as the coke drum expands and contracts during the temperature changes experienced by the coke drum during the delayed coking processes. The described connecting system securely supports the coke drum and prevents tipping of the drum, while allowing thermal contraction and expansion without undue stress to the support system, skirt or drum.
2. Background and Related Art
Many oil refineries recover valuable products from the heavy residual hydrocarbons (commonly referred to as resid or residuum) that remain following initial refining by a thermal cracking process known as delayed coking. The processing of crude oil into gasoline, diesel fuel, lubricants, and the like, as well as many other petroleum-refining operations, produces byproducts. The value of these byproducts can be substantially increased when they are processed by “destructive distillation.” During the process of destructive distillation, a portion of the byproducts is converted to usable hydrocarbon products. The remainder is transformed into a solid carbon product called coke. In the refining industry, this process is commonly known as delayed coking.
Generally, the delayed coking process involves heating the heavy hydrocarbon feed from a fractionation unit and then pumping the heated heavy feed into a large steel vessel commonly known as a coke drum. The nongaseous portion of the heated heavy feed settles out in the coke vessel where the combined effect of retention time and temperature causes the formation of coke. Vapors from the top of the coke vessel are returned to the fractionation unit for further processing into desired light hydrocarbon products. The operating conditions of delayed coking can be quite severe. Heavy feed input temperature may vary between 800 degrees Fahrenheit and 1000 degrees Fahrenheit.
Coke drums are typically large, cylindrical vessels commonly 19 to 30 feet in diameter and up to 120 feet tall having a top head and a funnel shaped bottom portion fitted with a bottom head and are usually present in pairs so that they can be operated alternately. The size, shape, and configuration of the coke drum may vary considerably from one installation to another. Coke is formed and accumulates in the vessel until it is filled to a safe margin, at which time the heated feed is switched to the empty “sister” coke vessel. This use of multiple coke drums enables the refinery to operate the fired heater and fractionation tower continuously. Thus, while one coke vessel is being filled with heated residual material, the other vessel is being cooled and cleared of coke (between 500 and 1200 tons) formed in the vessel during the previous recovery cycle. The full vessel is isolated, steamed to remove hydrocarbon vapors, cooled by filling with water, drained, opened, and the coke is drilled out with a water jet for removal out the bottom of the drum. The drums typically operate on a cycle, switching every 10 to 30 hours.
Coke removal begins with a quench step in which steam and then water are introduced into the coke-filled vessel to complete the recovery of volatile, light hydrocarbons and to cool the mass of coke. The vessel is drained, vented to atmospheric pressure, then opened at the bottom for removal of the coke. Removal is typically achieved using a drill bit fed my high pressure water directed through a jet or jets that cut the coke into small pieces which fall out the opened bottom of the coke drum. Once the coke has been removed, the drum is closed, warmed-up, and placed on stand-by, ready to repeat the 10- to 30-hour cycle.
Coke drums are largely vertical, with heights from three to four times their diameters. This large height/diameter ratio makes the coking drums susceptible to tipping due to forces such as those from strong winds, seismic activity, and piping attached to the drum. Further compounding this problem, the coke drums must be elevated to some extent to allow room underneath the coke drums for the dislodged coke to fall out and be removed during the decoking process. This increases the susceptibility of the coke drums to winds and other forces.
A typical coke drum is supported by a skirt which is welded to a lower portion of the drum. The skirt must support the weight of the drum, the coke formed in the drum and the water used to quench the drum. The skirt of the coke drum is typically bolted to a reinforced concrete base that provides the fixed support structure for the drum. This is problematic, however, for the cyclical decoking process subjects the large and heavy coke drum to frequent large temperature fluctuations which cause the drum to expand and contract. The drum is circumscribed by the skirt which expands and contracts at a rate different than the drum. The portion of the skirt that extends outwardly from the drum and which is supported by the supporting structures undergoes stresses often referred to as hoop stress. This can often be exacerbated as the skirt is insulated near the drum and not insulated in the areas farthest away from the drum. By constraining the expansion of the drum, the stresses in the skirt welded connection are incurred both during expansion and contraction of the drum. Some studies suggest that the weld between the skirt and the drum begins to fail from low cycle fatigue at peak stress locations within a few hundred cycles. Stress also occurs in the drum, the bolts and the concrete to which the drum is bolted. The failure of the system securing the coke drum to the concrete base may be gradual, difficult to monitor and costly to inspect.
Recent trends in the coking industry have elevated skirt failure concerns. Economic pressures have encouraged refineries to reduce the cycle times so that more coke may be produced in a given period. Faster production necessitates faster drum quenching causing more rapid cooling of the drum wall causing more stresses on the skirt connection.